X-ray intensifying screens are generally used in conjunction with silver halide photographic films and serve to enhance the image formed on that film. Phosphors, which are the active component of X-ray screens, are legion in number and include the tungstates, the oxysulfides and oxybromides among others.
Particularly efficient phosphors which may be used in an X-ray intensifying screen are the tantalates disclosed in U.S. Pat. No. 4,225,623. These phosphors are based on yttrium, lutetium and gadolinium tantalates of the M' monoclinic form, which may be activated with niobium or other rare earths, such as terbium and thulium, for example, as more fully described therein. Such phosphors are prepared by mixing the phosphor precursor materials and firing the mixture to form the phosphor crystal lattice itself. During the firing step, it is often beneficial to use a flux which usually forms a partial liquid at the elevated firing temperatures. Thus, the flux can be thought of as a fluid in which the various component parts of the phosphor precursor react to form the crystalline phosphor.
The average particle size of the phosphor effects the characteristics of the phosphor and, hence, the radiographic images. Generally speaking, it is desirable to use phosphor with a small average particle size in radiographic applications which the require highest possible resolution (e.g., mammography) and a large average particle size phosphor for procedures in which patient exposure must be minimized (e.g., serial angiography). In general, larger phosphor particles absorb more X-rays and emit higher quantities of long wavelength radiation than smaller particles under the same radiation. Thus, larger particles are faster, resulting in less patient exposure.
Theoretically, larger particles should give lower levels of quantum mottle or improved signal-to-noise ratio than smaller particles due to less light spread in the phosphor layer of the intensifying screen. These benefits are generally not observed, however, because the improvement in mottle is masked by increased phosphor X-ray-to-light conversion which requires less X-ray stimulation and thus lower signal levels. It is expected that improvements in signal-to-noise performance would be obtained (thus improving the diagnostic quality of radiographs) if large particles were produced with X-ray-to-light conversion equal to that of smaller particles.
Also, larger particles pack less efficiently than smaller particles when coated in an intensifying screen. To obtain equal phosphor coating weights, larger particles must be coated as a thicker layer than smaller particles. Packing efficiency is also related to dry bulk density of the phosphor, with higher dry bulk density resulting in thinner phosphor layers. As known in the art, thicker phosphor layers lead to increased image spread which is deleterious to resolution.
There has been a long felt need in the art to provide the practitioner with a means of systematically controlling particle size and bulk density in the phosphor make process.